Prepared for a Dipterists Forum workshop in 2004, keys to Nerioidea & Diopsoidea seem to have proved useful. They have been updated a little since then, notably in 2008 when a number of attendees at a similar workshop (different topic) took specimens of Imantimyia/Loxocera from which was developed the list of features described on this site. The keys are sufficient for UK species though they are poor for Chamaepsila, a difficult group.
Open access & out of copyright keys
Other published keys are suggested in the Summary to each taxon. The majority of the papers cited are Open Access. Please exercise caution with the older keys, there have been a number of revisions since they were published and they may not be at all reliable: